1. Field of the Invention
The invention concerns aqueous ink and use thereof.
2. Description of the Related Art
Aqueous inks are used in ink jet printing or color jet printing, in which droplets of an ink or coloring agent are applied to the paper which is to be printed upon. For that purpose it is possible to use either a liquid ink in which the dyestuff or dyestuffs are dissolved in a solvent or coloring agent which is solid at ambient temperature and which is melted thereon. To produce the printed image in droplet form the liquid ink is metered and deflected by electrostatic or magnetic fields or piezoelectric forces. Thus for example in the piezoelectric art, the droplets are produced by piezoelectric forces. In the bubble jet art the droplets are deflected on to the paper by the pressure which occurs upon vaporization of the solvent.
The inks used in such systems must penetrate immediately into the material to be printed upon (such penetration is also referred to as ink absorption), in order that it is not smeared. That is achieved by virtue of the choice of solvents. Normally inks contain a solvent mixture of solvents with various properties to achieve desired effects. Those solvents must be compatible with each other, in other words, they should be soluble or emulsifiable in the basic solvent, so that no phase separation effect occurs. If for ecological and economic reasons water is the solvent of choice, the other solvents used must be water-soluble or emulsifiable. So that the ink is rapidly absorbed by the material to which the printing is to be applied, the ink contains a penetration agent which accelerates the penetration effect. If the time which a given amount of solvent, for example water, requires to penetrate into a substrate is plotted in a chart in dependence on the amount of penetration agent, that gives a curve with a minimum for the penetration time, which is referred to as tmin. In order to achieve that minimum penetration time, the requirement is for a minimum amount of a penetration agent, which is referred to as the limit concentration. The smaller the value of tmin and the lower the limit concentration, the correspondingly more advantageous is the penetration agent in question.
The minimum penetration time is influenced by other constituents which are present in the composition, in particular by the other solvents present. For example inks usually contain in the solvent mixture one or more constituents that have a high boiling point and which are intended to prevent the ink from drying up at the nozzles and causing nozzle encrustation. This high-boiling point solvent which, as discussed above, must be compatible with the basic solvent, and therefore, water-soluble or emulsifiable for aqueous inks, modifies the penetration characteristics of the ink, in other words the limit concentration becomes lower but the penetration times become longer, more specifically, to an increasing degree, the more the high-boiling solvent is polar and thus water-soluble.
With rising polarity, however, the penetration capability of the ink falls as that depends inter alia on the wetting capability in relation to the material to be printed thereon and nonpolar solvents. For example, benzene can wet a paper surface better than polar solvents, such as, water. It is therefore necessary to find a high-boiling solvent which on the one hand is sufficiently polar for it to be water-soluble, while on the other hand it does not excessively impede penetration.
The known aqueous inks often use a solvent mixture that includes glycol as a high-boiling gent and a glycolether as a penetration promoter. Those known ink formulations, however, suffer from the disadvantage that, in relation to many kinds of paper, for example when using recycled paper and slightly sized paper, they cause the ink to run severely in terms of width and they cause the ink to fray out along the paper fibers (also referred to as staining or feathering) which has an adverse effect on the print image.
A further disadvantage of these known ink formulations is that they give characters of poor contrast as the major part of the dyestuff is transported into the paper by the solvent and for that reason only a very small part of the dyestuff remains visibly at the surface of the paper. These known inks are therefore less suitable for printing characters and bar-codes which are to be machine-readable.
An area of use in which machine-readability and, because of the high through-put rate, short penetration times are important factors, is the field of franking machines. Franking machine systems are primarily operated with red or red-fluorescing inks, by virtue of the postal regulations. They also did not need to be machine-readable. When using fluorescent inks, the only check was to ascertain whether a red fluorescence of between 580 and 640 nm is to be detected, with an excitation wavelength of 254 nm.
In the course of automation, more powerful technology is now being used in the areas of checking, franking, sorting and postal delivery. The use of one-dimensional and/or two-dimensional bar codes in franking machines is necessary for that purpose, for clearly identifying the franking. However, they cannot be produced in machine-readable form with the inks which have been used in franking machines.
The properties required for ink jet printing cannot be optimally achieved with the previously known inks. Known inks either have excessively long penetration times; do not enjoy adequate penetration capability, for example such inks as are known from U.S. Pat. No. 5,106,416; or produce feathered prints of low contrast, for example such inks as are known from U.S. Pat. No. 5,630,867 and U.S. Pat. No. 5,674,314. A further example of a known ink is disclosed in U.S. Pat. No. 5,681,381 and U.S. Pat. No. 5,674,314.
An object of the invention was to provide an aqueous ink with good penetration capability, with which a high-contrast image of high resolution can be produced on a color or ink jet printer, without the characters suffering from feathering in the printing thereof. In particular an object of the invention was to provide an ink which is suitable for printing machine-readable characters and bar codes with a color or ink jet printer and which can also be used for franking machines.
That object is attained by an aqueous ink which contains one or more dyestuffs, at least one alkanol with a straight or branched C3-C8-alkyl chain and 2 to 4 hydroxyl groups, at least one C1-C6-alkyl ester of a xe2x88x9d-hydroxycarboxylic acid of the formula Rxe2x80x94C(OH)xe2x80x94COOH, wherein R is H or a straight or branched C1-C6-alkyl residue and possibly further adjuvants, wherein the balance is water.